Regardless of climatic conditions, commercial, institutional and industrial buildings are air conditioned due to the steady escalation of internal heat gains. Depending on the type, building occupacy and climate, the cost of air conditioning systems vary from 10 to 30 percent of total construction costs with the cooling plant accounting for more than half of the total air conditioning system cost.
In the various industrial sectors many process and manufacturing operations involve process cooling to maintain the plant performance, with the required cooling system representing a significant portion of the overall plant capital, amortization and operating costs
Combination of unit operations used in cooling systems may include heat exchange, indirect and direct evaporative cooling, mechanical vapour compression refrigeration, absorption refrigeration and adsorption or absorption dehumidification. Alternative combinations of these operations are being sought by users seeking lower operating costs and by utilities facing increasing demand for electric power used to operate these facilities.
One prior art system recently developed by Cargo Caire Engineering Corp. (Gas Research Institute Digest, Vol.8. No. 4., Winter 1985/86, p.30) uses a combination of adsorption dehumidification, vapour compression refrigeration, evaporation and heat exchange operations to dehumidify and cool the outdoor air for use in air conditioning of space. In the adsorption dehumidification stage of the treatment the prior art system uses a rotating desiccant wheel concept with a waste heat recovery heat exchanger to improve the efficiency of the adsorption step and uses natural gas as fuel in preheating the air used to reactivate spent desiccant.
Another prior art system described by U.S. Pat. No. 4,222,244 issued to G. Meckler, uses a combination of desiccant rotating wheel dehumidification, vapor compression refrigeration and heat exchange operations to cool air for air conditioning and uses waste heat energy and solar energy to reactivate spent desiccant.
The engineering principles of adsorption are well known and documented and the operation is effectively used in many applications involving recovery of solvents, separation of petrochemicals separation of oxygen and nitrogen from air, removing of toxic gases from gas streams and removing of moisture from liquid and gaseous products. Similarly, principles of evaporative cooling are well known and the operation is broadly used including air conditioning of buildings.
Adsorption systems used in dehumidification of air may use an adsorbent-desiccant structure which may be a fixed bed, a moving bed, or a rotating bed which may be a disc, drum or wheel, and containing a suitable adsorbent-desiccant or which structure may be filled with an inert porous solid material impregnated with liquid absorbent such as lithium chloride.
The fixed bed systems may use a single, two, or more fixed beds of granular sorbent, and are provided with a quick closing valve arrangement permitting the cycling of the treated air and the regenerant streams between individual beds.
The rotating systems are equipped with a suitable rotating mechanism to rotate the adsorbent bed between the processed air and regenerant streams which are separated from each other by seals to prevent mixing.
All systems are equipped with an adsorbent reactivation subsystem. The reactivation of spent adsorbent may be either by the heat energy of the regenerant, by evacuation of the adsorbent bed, by heating of the bed, by an inert dry gas or by some combination thereof.
It is well known that the amount of moisture removed from air by adsorption depends on properties of the adsorbent, on the temperature of adsorbent during adsorption, on the temperature, pressure and humidity of the treated air, and on the contact time of the treated air with the adsorbent.
It is also known that the effectiveness of reactivation of spent adsorbent depends on the rate and magnitude of change of the moisture-adsorbent equilibrium, which depends on the reactivation temperature, pressure, vapour pressure of moisture in regenerant and heat capacity of adsorbent.
It is also known that the released adsorption heat and the residual heat retained by adsorbent from the preceded desorption period cause an increase in the temperature of the adsorbent bed and of the treated air and inhibit the effectiveness of dehumidification.
It is also known that adiabatic cooling of dry air in direct contact with water reduces the treated air temperature and that the resulting air-water temperature depends on the dry bulb temperature and initial humidity of the air and on the effectiveness of the evaporator. And it is also known that the quality of the outdoor air depends on weather conditions and location.
While all the above process principles are known and well documented, it remains, that the current art desiccant cooling systems can not produce the required low temperatures of the treated air without a considerable amount of useful energy used up in the reactivation of the spent desiccant.
It is therefore the object of the present invention to provide an apparatus and a method for cooling of air for use in air conditioning of buildings without the need for the useful energy required by the prior art for the reactivation of the spent desiccant.